When the intention is to manufacture pure metal such as copper, hydrometallurgical methods such as electrolytic refining or recovery are used. The electrowinning and electrorefining processes are current methods to recover the metals, such as copper, zinc, cobalt or nickel. In electrolytic refining, impure metal anodes are dissolved electrochemically, and the metal dissolved from them is reduced onto the cathode. In electrolytic recovery, the metal is reduced directly from the electrolytic solution. The cathodes used in the process can be starter sheets made of the metal to be reduced, or permanent cathodes made of stainless steel, for example. A transition to the use of permanent cathodes has been the prevailing trend at electrolytic plants for a long time, and in practice, e.g. all new copper electrolysis processes are based on this technology.
A permanent cathode is formed of a cathode plate and an attached suspension bar using which the cathode is suspended in the electrolytic bath. The deposited metal can be mechanically stripped from the surfaces of permanent cathode plate, and the permanent cathodes can be reused. Permanent cathodes can be used in both electrolytic refining and recovery of metals. The corrosion resistance of the steel grade used as a permanent cathode plate in the electrolyte is not enough to guarantee that the properties required of the cathode are fulfilled. Substantial attention must be paid to the adhesion properties of the cathode plate surface. The surface properties of a permanent cathode plate must be appropriate so that the depositing metal does not spontaneously strip off from the surface during the electrolytic process but adheres sufficiently, however not preventing the deposited metal from being removed using a stripping machine, for example.
The most important properties required of a permanent cathode plate include corrosion resistance, straightness and surface properties with regard to the adhesion and removability (strippability) of the deposited metal.
During years in operation the permanent cathode plates deteriorate by the chemical (corrosion) and mechanical (bending and hammering during stripping) effects to such a condition that the surface properties may not any more fulfill the requirements of sufficient adhesion and removability. In operation, cruds and mottles are formed on the surfaces of the permanent cathode plate and the surface quality deteriorates during lifetime due to scratches and dents generated in use and corrosion. Therefore the permanent cathode does not any more function optimally and adhesion problems may occur.
So far, the only solution to prolong the lifetime of the permanent cathodes has been the maintenance of the permanent cathode plates by subjecting them to periodical repair where the accumulated crud and scratches are removed from the surfaces by grinding and the edge insulation is replaced. The permanent cathode plate may also be straightened if required. The problem with the current method is that, in practice, it has proved that such a treatment solves the problem only momentarily.
It is known, that in addition to the macro roughness of the surface, which is a commonly measured characteristics and which is changed in grinding, also the characteristics of the grain boundaries have a significant role for the adhesion and strippability of the deposited metal because the grain boundaries in micro scale serve as adhesion points for the depositing metal. The depth and width of the grain boundaries must be in a certain relation to each other so that the depositing metal adheres sufficiently but not too tightly to the surface of the permanent cathode plate. A prior art document WO 2012/175803 A2 discloses preferable grain boundary dimensions for permanent cathode plates.
In operation, impurities and cruds are precipitated on the grain boundaries and on the grain interiors and also the corrosion changes the micro structure so that the grain boundaries become oversize, i.e. overly deep and/or wide, whereby optimal surface characteristics are lost.
Examples of the deteriorated surfaces of the permanent cathode plates are shown in FIGS. 1 to 4. FIG. 1 shows how a used and deteriorated permanent cathode plate looks like visually seen by eye. The plate is severely mottled. FIG. 2 shows a microscopic view of the used and deteriorated permanent cathode plate showing the copper arsenide crud covering the surface. Grain boundaries under the crud are barely visible. FIG. 3 shows a microscopic view of the used and deteriorated permanent cathode plate showing black and white crud on the surface. Grain boundaries under the crud are barely visible. FIG. 4 shows a microscopic view of the used permanent cathode plate surface after the crud has been removed. Pitting corrosion on the grain boundaries can be seen making the grain boundaries overly wide and deep and non-optimal with respect to adhesion and strippability.
The currently available maintenance by grinding affects only the macro roughness of the surface of the permanent cathode plate, said macro roughness having only a secondary role to the functionality of the permanent cathode plate. Further, the microscopic sharp formations on surface caused by grinding are disadvantageous from the point of view of crud accumulation, corrosion resistance and current distribution which may explain the rapid degradation of the quality of the merely ground surface in use. Therefore, prolonging of the lifetime of the permanent cathodes only by the currently available method does not provide a durable and long-lasting result.